This invention relates to the field of producing shaped articles from polyvinyl halide polymers and, more particularly, to improved processing aids for use in molding, calendering, extrusion or other mechanical processing of polyvinyl halides.
Polyvinyl halides such as polyvinyl chloride (PVC) possess service properties which commend their use in a variety of applications. The electrical resistivity of PVC has led to widespread use as a wire and cable insulation. A substantial industry in PVC pipe and conduit has been made possible by the mechanical strength of the polymer and its resistance to water and solvents. A very substantial volume of PVC is also consumed in the manufacture of film, foam and a variety of products produced by extrusion and injection molding.
Because of the hardness of vinyl chloride homopolymer and its susceptibility to thermal degradation, mechanical processing of the homopolymer has required substantial use of plasticizers to render it more readily workable, and heat stabilizers to prevent degradation at mechanical processing temperatures. A variety of other additives including lubricants, fillers, and impact modifiers are conventionally compounded with vinyl chloride homopolymer to provide compositions suited for use in calendering, injection molding, extrusion and other mechanical working processes.
Although plasticizers soften PVC, increase its flexibility, and generally facilitate its processing, the presence of plasticizers may alter the mechanical strength of the polymer and the shaped articles produced therefrom. If blending and processing are not properly controlled, or if conditions of use are severe, plasticizers may tend to bleed or weep from the surfaces of the PVC product. There has thus been a need in the art for means facilitating the processing of polyvinyl halide polymers while minimizing adverse effects on the end product.
In response to this need, various polymer processing aids (PPAs) have been developed. These PPAs are typically polymeric materials that are blended with the polyvinyl halide polymer prior to processing. They are intended to facilitate processing by promoting rapid fusion, lowering melt viscosity, improving melt strength, improving surface quality and lowering melt temperature, without diminishing the physical properties of the end product produced from the polyvinyl halide. Among the commercially available PPAs are various copolymers that are predominantly constituted of methyl methacrylate, for example, Rohm & Haas's "K-120-N", or of styrene or substituted styrene, for example, "Amoco 18." While both of these types are useful processing aids, a need has remained in the art for PPAs which provide rapid fusion while exhibiting a high compatibility with polyvinyl halide polymers and not reducing the heat deflection temperature of shaped articles produced from polyvinyl halide blends containing the PPA. Certain of the commercial processing aids also tend to cause swelling of extruded PVC as it emanates from the die. There has, thus, been a need for PPAs which provide processability with minimum die swell. Further continuing needs have persisted for PPAs which preserve the clarity of sheets and film and which provide the lowest feasible melt viscosity so as to maximize productivity and minimize power consumption during mechanical processing of the vinyl halide polymer.
Jennings U.S. Pat. No. 2,646,417 describes vinyl halide compositions which contain a high molecular weight interpolymer of styrene and acrylonitrile as a processing aid. Optionally the styrene/acrylonitrile copolymer can contain up to 20% by weight of another monoolefinic monomeric material. The third monomer may be selected from a long list of vinyl esters, acrylic and .alpha.-alkyl acrylic acids, their alkyl esters, their amides, and their nitriles, vinyl alkyl ethers and ketones, and alkyl esters of maleic and fumaric acid. The exemplary disclosure describes a copolymer containing 75% styrene and 25% acrylonitrile, and a terpolymer containing 65% styrene, 25% acrylonitrile and 10% ethyl acrylate, as processing aids for the vinyl halide polymer.
Barkhuff et al. U.S. Pat. No. 3,073,791 describes an emulsion interpolymer of an alkyl ester of fumaric acid, an unsaturated acid such as acrylic, cinnamic, atropic or crotonic, a hardening comonomer such as styrene, and optionally, an unsaturated nitrile. The product is a film former described as useful in textile finishing.
Wesp et al. U.S. Pat. No. 2,851,444 discloses terpolymers of styrene, dialkyl fumarate and at least 2% of a third monomer which may be of a different dialkyl fumarate. Exemplified are terpolymers of styrene/methacrylic acid/diethyl fumarate and vinyltoluene/methacrylonitrile/methacrylic acid. The disclosure is primarily concerned with the preparation of clear terpolymers which may be selected from a narrow range of compositions along a line joining binary polymerization azeotropes on a three component monomer composition diagram. Polymerization azeotropes are described as monomer compositions containing such relative proportions of monomers that, when subjected to batchwise bulk polymerization, copolymers are formed having the same composition as the monomer mixture. The terpolymers described by Wesp et al. are film forming polymers that can be blended with other polymers, plasticizers, solvents, fillers, pigments, dyes, stabilizers and the like.